This program is intended to elucidate the laws that regulate the expression of different immunoglobulin classes on bone-marrow derived (B) lymphocytes with particular attention to the problem of simultaneous of two different immunoglobulin by the same cell. The physiological function of the membrane immunoglobulin will be studied in its relationships to the stimulation by antigens or mitogens towards cell proliferation and/or maturation. The possible connections between membrane immunoglobulin expression and the different stages of B lymphocyte development up to the level of memory cells will be studied. A direct approach of mitogen focusing, as well as different techniques of cell separation will be used to study the "switches" from the synthesis of one immunoglobulin class to another, with particular attention to the switches that involve the IgD class.